Fill a medium sized pot with two inches of water and the vinegar. Bring to a "slow boil" so bubbles break the surface as little as possible. but it's still boiling. Find a glass or metal mixing bowl that will fit on top of that pot without sitting in the water. take it off the pot.
Split an English muffin and butter and toast it to desired toastyness. Place face up on plates and top with spinach and tomato slices.
To make the hollandaise sauce, In the mixing bowl, whisk together egg yolks, lemon juice, pepper, and mustard. Melt butter in a microwave checking it every five minutes until fully melted. Place the bowl back on top of your boiling pot and gradually whisk butter into the yolk mixture. Continue whisking over low heat until sauce is thickened, about 8 minutes. Remove from heat and keep warm.
Place 2 eggs in separate shallow dishes. With a spoon stir the water to make a gentle spinning whirlpool then dip one of the dishes with the egg in it into the water and turn the dish until the egg falls out. give the water another spin to form the egg whites around the yolk. Repeat with the other egg. Keep the eggs gently moving in the water for 3 minutes then remove with a slotted spoon and plate on top of the tomatoes.
Put the hollandaise sauce back on the pot to heat it back up. Add water a teaspoon at a time if it has thickened up too much. It should be smooth and creamy. Ladle over the eggs then top with the crumbled feta. If you want to get fancy pierce the poached egg right before serving.